Abstract
Haldane’s dilemma refers to the fact that selection acting at many loci could incur an unsustainable reproductive cost. This is because natural selection involves the removal of disfavored genotypes from the population and their replacement with selectively favored genotypes. Consequently, it requires a certain amount of reproductive excess to maintain a constant population size. The required reproductive excess is referred to as the cost of selection. It is generally assumed that this cost is cumulative when several genetic loci are subject to independent selection. Consequently, even if the amount of reproductive excess is sufficient to allow allelic replacement at a single locus, it has been argued that it may not be sufficient to allow many genes to undergo independent selective replacement simultaneously. This cumulative cost of multi-locus selection has been seen as a potentially limiting factor on the rate of adaptive evolution. In this paper we show that Haldane’s arguments for the accumulation of costs are valid for a clonally reproducing population of asexual genotypes. But recombination, in a randomly mating sexual population, eliminates the accumulation of costs over multiple loci. Thus, sex removes a perceived reproductive constraint on the rate of adaptive evolution. These results are particularly relevant to recent evolutionary models of adaptation based on the standing genetic variation.